You are correct that they thinned out their line. They must think that’s an acceptable risk. Even top military advisors aren’t sure what the goals of the assault are (they have good guesses), so assuming we know if it’s worth it or not is really dumb.

The best reason I’ve seen I think is that Russia has to retake this land, no matter the cost. If Ukraine can come in and hold ground that’s a huge PR loss for Russia, who tells their citizens it can’t happen and they’re so strong. We’re coming up to winter. Historically, Ukraine does much better in defence, but also Russia stops assaulting during the winter. Holding this land in Russia creates a point that Russia has to attack, even in the winter.

There are also many other reasons, such as they’re working on a very large logistics cutoff by destroying some bridges that are now the only way for Russia to access after the push. Any way they can put more strain on Russia’s supply line is huge. Their soldiers were already seemingly not super well supplied. It also creates situations where Russia is redeploying to counter this push and Ukraine can use its extremely accurate HIMARS to get a lot of kills while they’re grouped up in trucks.

Basically, there are a lot of potential advantages, and the cost seems to have been minimal, mostly opporutiny cost. We can only trust their military leaders aren’t idiots, which they don’t seem to be. Making assumptions on our own, or even listening to military experts, is not going to be accurate. Everyone is making guesses at best.